It is known to transmit power over data lines to power remote equipment. Power Over Ethernet (PoE) is an example of one such system. In PoE, limited power is transmitted to Ethernet-connected equipment (e.g., VoIP telephones, WLAN transmitters, security cameras, etc.) from an Ethernet switch. DC power from the switch is transmitted over two sets of twisted pair wires in the standard CAT-5 cabling. The same two sets of twisted pair wires may also transmit differential data signals, since the DC common mode voltage does not affect the data. In this way, the need for providing any external power source for the “Powered Devices” (PDs) can be eliminated. The standards for PoE are set out in IEEE 802.3, incorporated herein by reference.
Providing power over data lines is applicable to other existing systems and future systems. For example, electronic equipment in automobiles will increasingly benefit from power to the equipment being provided over the data lines to reduce wiring. Various new systems using power over data lines may be standardized by the IEEE or other groups.
Such systems using power over data lines may or may not require handshaking protocols. The present invention applies to systems requiring some sort of indication from the PD that the full PoE voltage is to be applied to the data lines.
Although the present invention may be applied to any system using power over data lines, a typical PoE system will be described as an example.
FIG. 1 represents a typical Ethernet system using PoE. In the example of FIG. 1, a “Power Sourcing Equipment” (PSE) 12 may be any Ethernet device that supplies power and data to a PD. The PSE 12 and PD 14 are typically connected via a standard CAT-5 cable terminated with the standard Ethernet 8-pin (four twisted pairs) RJ45 connector. Only two of the twisted pairs are typically needed for PoE and data.
The PSE 12 is typically powered by the mains voltage (120 VAC) and uses either an external or internal voltage converter 16 to generate a DC voltage between 44-57 volts. The PoE standards require the PSE to supply a minimum of 37 volts at the PD. The voltage drop along the cable increases with distance.
Two of the twisted pairs 18 and 20 are assigned to carry the PoE power, and these pairs may also carry differential data. The remaining two pairs are also shown. All pairs in use are terminated at the PD 14 by transformers, such as transformers 22 and 24. It is assumed that the twisted pair 18 provides 44 volts and the twisted pair 20 is connected to ground. A connection is made to the center tap of transformers 22 and 24 to provide the 44 volts to the PD 14. Since the DC voltage is common mode, it does not affect the differential data. Other conventional termination circuitry is also included in the termination block 25, such as polarity correction circuitry (a diode bridge) downstream from the transformers, but is not relevant to the present invention.
The 44 volts is applied to a DC-DC converter 26 for converting the voltage to any voltage or voltages required by the PD 14. The load 28 (e.g., a security camera) is powered by the converter 26 and communicates with the PSE 12 via the twisted wire pairs.
The IEEE standards require certain low current handshaking procedures between the PSE 12 and PD 14 in order to detect the presence of a PoE-powered device and in order to convey the pertinent characteristics of the PSE 12 and PD 14 prior to the PSE 12 making the full power available to the PD 14. The detection/classification circuit 29 controls the handshaking routine, and may be a state machine, a processor, or any other suitable control circuit. The PSE 12 also contains a circuit for carrying out the handshaking routine. The circuits for carrying out the handshaking routine are well-known ICs.
Below is a simplified summary of the handshaking routine between the PSE 12 and the PD 14.
When the PSE 12 is first connected to the PD 14 via an Ethernet cable, the PSE 12 interrogates the PD 14 to determine if it is PoE-enabled. This period is termed the detection phase. During the detection phase, the PSE 12 applies a first current limited voltage for a fixed interval to the PD 14, via the twisted wire pairs 18 and 20, and then applies a second current limited voltage for a fixed interval, while looking for a characteristic impedance of the PD 14 (about 25k ohms) by detecting the resulting current. If the correct impedance is not detected, the PSE 12 assumes that the load is not PoE-enabled and shuts down the PoE generating end. The system then operates as a standard Ethernet connection.
If the signature impedance is detected, the PSE 12 moves on to an optional classification phase. The PSE 12 ramps up the voltage to the PD 14. The PSE 12 generates either one pulse (indicating it is a Type 1 PSE) or two pulses (indicating it is a Type 2 PSE). The PD 14 responds to the classification pulses with certain current levels to identify whether the PD 14 is Type 1 or Type 2. A Type 1 PD requires less than 13 W. A Type 2 PD requires up to a maximum of 25.5 W. Various classes (e.g., five classes), each associated with a maximum average current level and a maximum instantaneous current level, within these types may also be identified. A classification resistance may be used. The PSE 12 then may use this power demand information to determine if it can supply the required power to the PD 14, and the PD 14 uses the information to determine if it can fully operate with the PSE 12. There are maximum time windows for the detection and classification phases (e.g., 500 ms).
Other types of detection and classification routines and standards may be implemented in the future.
On completion of the detection and classification phases, the PSE 12 ramps its output voltage above 42 V. Once an under-voltage lockout (UVLO) threshold has been detected at the PD 14, an internal FET is turned on to connect the full PoE voltage to the converter 26, and the converter 26 supplies a regulated DC voltage to the load 28. At this point, the PD 14 begins to operate normally, and it continues to operate normally as long as the input voltage remains above a required level.
As seen, the handshaking operation requires multiple low-current operations that take time to conduct. Further, the PD must contain a PoE controller IC for generating the various handshaking signals, and such an IC adds cost to each PD. Further, the 25k ohm resistor adds to the current draw by the PD during normal operation.
What is needed is a new technique to identify to the PSE that the PD is PoE enabled and to identify the power requirements of the PD, such as whether the PD is a Type 1 or Type 2 device, where the technique does not suffer from the drawbacks of the prior art technique.